1. Field of the Invention
The present invention relates to a pressure sensor and, in particular, to a pressure sensor employing a field emission cold cathode device that is usable in a radioactive irradiation environment or a high temperature environment.
2. Description of the Related Art
As a conventional small, high-sensitive pressure sensor, a diffusion-type semiconductor pressure sensor is known. Its construction is exemplified in, for example, Sensor Cyclopedia, chief ed. Kiyoshi TAKAHASHI (Asakura Shoten Co., 1991). An example of the diffusion-type semiconductor pressure sensor will now be described with reference to FIG. 1.
FIG. 1 illustrates a pressure-reception diaphragm 1 consisting of processed silicon monocrystal. If the pressure-reception diaphragm 1 is made of, for example, n-type silicon, boron is diffused on a portion of the pressure-reception diaphragm 1 where distortion is detected and a p-type gauge resistance 2 is thereby formed. The pressure-reception diaphragm 1 is provided, through a connecting portion 4, on base 5, to which a pressure introduction pipe 6 is connected.
If pressure is introduced into the pressure-reception diaphragm 1 through the pressure introduction pipe 6, the pressure-reception diaphragm 1 is deformed according to the pressure introduced. The resistance value of the p-type gauge resistance formed on the portion of the pressure-reception diaphragm 1, where distortion is detected, varies with the deformation. Pressure can be thus detected by measuring a variation in resistance.
However, the pressure sensor employing semiconductor elements of this kind has such a problem as to be unable to operate in a high-temperature environment or a radioactive irradiation environment due to its vulnerability to high temperature and radiation. For the diffusion-type semiconductor pressure sensor, for instance, temperatures at which there is no change in its operation characteristics usually range around 0.degree. C. to 50.degree. C. and temperatures at which the sensor is operable range around -20.degree. C. to 100.degree. C. In respect of radiation, it does not operate properly if the accumulated dose of gamma radiation reaches around 10.sup.5 Roentgen. In the containment vessel of an atomic reactor, the accumulated dose of gamma radiation is 10.sup.4 Roentgen per hour. Considering this, it is inappropriate to measure pressure, using a conventional pressure sensor employing semiconductor elements in this environment.
It is of quite importance to grasp pressure variations in the containment vessel when an accident happens, in view of preventing the accident from spreading or of investigating its cause later. At this time, an accumulated dose might be well above normal operation level. It is, in particular, inappropriate to use the conventional semiconductor elements for measuring pressure at the time of the accident even for a short period of time.
As mentioned above, problems arise in the conventional diffusion-type semiconductor sensor. Namely, charging carriers within the semiconductor elements are sensitive to temperature and radioactive irradiation, and radioactive irradiation causes defects relatively easily. Therefore, the sensor, which is vulnerable to high-temperature or radiation, does not operate properly in the environment.